Sand filters (SSFs) certainly are a simple but efficient method to remove turbidity and pathogens from water. They include a basin with a level of fine sand (typically as much as 0.6 m deep) along with support gravel, above which an underdrain system ensures that clean water can flow out. They’re typically housed in reinforced concrete structures and the water level is normally kept below sand height. The sand surface acts as a habitat for a varied microbial community (Schmutzdecke) that is in charge of the biological removal of contaminants and pathogens. The microbial community also mechanically strains and chemically filters the water. Achtformpools
Pathogens are removed by heat, competition for food, predation and physical destruction of the cell walls (e.g. abrasion). Water flowing through the filter could be disinfected by chlorination in the final treatment step.
However, despite their robust and relatively inexpensive design, SSFs are not immune to bacterial contamination. Consequently, they must be regularly tested to make sure that they continue to do well. Typical tests involve the measurement of Heterotrophic Plate Counts (HPC) in filtered water and the isolation of bacteria from the sand filter influent and effluent.
Several studies demonstrate that SSFs generally achieve significant mean reductions in E. coli, enterococci, Clostridium perfringens spores and coliphages in the secondary effluent. However, HPC is just a very broad indicator of the microbial load in water, and thus only a partial picture of overall pathogen reduction by SSFs could be obtained.
To be able to gain more insight into the specific removal capacity of SSFs against particular pathogens, we challenged some slow bio-sand filters with two distinctly different plant pathogens: Phytophthora capsici and Fusarium oxysporum f. sp. Lycopersici, based on their phylogenetic differences (oomycete and fungus), their biology (cell walls of b-1,3 glucans in the oomycetes and chitin in the fungi) and their capability to spread via water.
In the experiment, each filter was operated in its normal mode for 12 weeks, then subjected to 1 week of simulated pump failure. In this period, daily inoculation with the two pathogens was suspended and samples were collected from the pre- and post-filter ports to gauge the elimination performance of the SSFs against both pathogens. The outcome reveal that the microbial communities in the SSFs are highly dynamic and respond rapidly to the changing conditions. However, the SSFs can reliably remove both Phytophthora and Fusarium. The removal rate is significantly higher for Phytophthora than for Fusarium. However, the elimination rate for both is significantly greater than in a control sample without any challenge. This indicates that the microbial adaptation of SSFs to a given pathogen is just a strong predictor of the removal efficiency. Consequently, the microbial community composition in slow bio-sand filters should be studied into account when selecting the filter sand particle size and thickness for a water treatment application. These records can be used to optimize the sand particle size and filtration rate for a maximum pathogen removal performance.
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